For advanced divers looking for a unique experience, Howe Sound is the only place in the world where glass sponge reefs known as bioherms are found at air-diveable depths. Bioherms were once thought to have gone extinct sometime in the Jurassic period and were only rediscovered alive in the late 1980s. Made completely of sponge, these huge reefs rise up from seemingly empty ocean floor and have become home to many other creatures.